With development of modern communications technologies, more communications standards emerge, and consequently, a signal of a communications standard interferes with a signal of another communications standard. This requires that a modern communications system should have a strong capability of suppressing an out-band signal. Based on the foregoing requirement, a filter having a desirable out-band suppression function is urgently required. In other approaches, many filters suppress a high-order harmonic wave in a base frequency signal by adding a transmission zero. In addition, some filters generate a wide stopband using a stepped impedance resonator (SIR). However, areas of the filters are large, and harmonic suppression capabilities of the filters also need to be improved.